I think Amity or Artemis are both great with your last name and follow your description of either kind of historic or pilgrim names. I think in USA Amity is more rare but it could just be by my area.
I don't think Pearl, Ivy or Celementine go great with your last name.

Sad you can't use Xanthe, I love that name. Pearl is a lovely name, but I don't think it works well with Irwin, and would avoid 'err' sounds.

Here are my suggestions:Thisbe (sigh... love this one as much as Xanthe). Has Greek and Shakespearean associations...Zenaida (there are several spellings/possible pronunciations on that, but such a lovely name), it means "life of Zeus."

A couple of other suggestions- how about Agate instead of Pearl for a gem-type name? It's a bit more under the radar than Ruby or Emerald...

What about Magnolia instead of Amaryllis? It has some great nickname options. It maybe doesn't have the mythological history of some of the other names, but it also stays away from the difficult territory that comes with that (a lot of nymphs run into trouble...).

I hope I can help you find a name as stunning as Iris. I've always found Iris to be gorgeous!

Antigone (daughter of Oedipus. Perhaps not the greatest meaning/story. But the name is extraodinarily pretty spoken.)
Astyoche/Astyocheia (name of many from Greek mythology.)
Euphrosyne (name of an Empress, princess, actress, saint, and more. The name is grounded in history and literature & art surrounds it.)Evadne (Greek like Ariadne, but more unusual. A daughter of Poseidon and bore Apollos son, Iamus ("son of the violets").
Henioche (means charioteer. found in Greek Mythology.) Iphigenia (daughter of Agamemnon & Clytemnestra. Agamemnon was forced to sacrifice her to allow ships to sail to Troy.)
Loveday (Old English name for children born on Loveday. It was used as a boys and girls name)
Orithyia (daughter of King Erechtheus of Athens. Was abducted by the North Wind and written of by Plato. Greek Mythology.)
Pelopia (Greek mythology: daughter of Thyestes, daughter of Pelias, daughter of Cycnus and one of the Niobid.)
Perimele (Greek myth: daughter of Hippodamas. Loved by Achelous and was pushed of a cliff for it. Achelous prayed to Poseidon to keep her safe. She floats on the waves and eventually turns into an island, known today as Perimele.)